The light weight and strength of magnesium and magnesium alloys makes products fashioned therefore highly desirable for use in manufacturing critical components of, for example, aircraft, terrestrial vehicles and electronic devices. One of the most significant disadvantages of magnesium and magnesium alloys is corrosion. Exposure to the elements causes magnesium and magnesium alloy surfaces to corrode rather quickly, corrosion that is both unesthetic and reduces strength.
There are many methods for improving the corrosion resistance of a magnesium and magnesium alloy workpiece by modifying the surface of the workpiece. It is generally accepted that the best corrosion resistance for magnesium and magnesium alloy surfaces is achieved by anodization. In anodization, a metal workpiece is used as an anode of an electrical circuit, the circuit including an electrolyte bath in which the workpiece is immersed. Depending on the properties of the current, bath temperature and the composition of the electrolyte bath, the surface of the workpiece is modified in various ways. Various solutions and additives are found in, for example: U.S. Pat. No. 4,023,986 (trihalogenated compound and a group 1b, 2, 3a, 4b, 5b, 6b and 8 metal and an alkarylamine); U.S. Pat. No. 4,184,926 (alkali metal silicate and alkali metal hydroxide solution); U.S. Pat. No. 4,551,211 (aluminate and alkali hydroxide and boron/sulfate/phenol/iodine solution); U.S. Pat. No. 4,620,904 (basic silicate and hydroxide and fluoride solution); U.S. Pat. No. 4,978,432 (basic pH with borate/sulfonate, phosphate and fluoride/chloride solution); U.S. Pat. No. 5,264,113 (basic pH with fluoride solution followed by basic with hydroxide, fluoride and silicate solution); U.S. Pat. No. 5,470,664 (neutral NH4F solution followed by basic hydroxide and fluoride/fluorosilicate and silicate solution); U.S. Pat. No. 5,792,335 (ammonia and phosphate solution with optional ammonium salts and optional peroxides); and U.S. Pat. No. 6,280,598 (various amines/ammonia and phosphate/fluoride with optional sealing agents).
Although anodization is effective in increasing corrosion resistance and the hardness of the surface, anodization is not perfect.
Anodized magnesium surface become very rough, with many pores caused by sparking during the anodization procedure. These pores trap humidity and other corrosion-inducing agents. Upon exposure to extreme conditions, humidity is trapped in the pores, leading to corrosion. The use of ammonia or amine in the solutions taught in U.S. Pat. No. 5,792,335 and U.S. Pat. No. 6,280,598 apparently reduces the extent of sparking, leading to smaller pores.
An additional disadvantage is that an anodized surface is electronically insulating. Thus anodization cannot be used in applications where an electrically conductive workpiece is desired. Applications where the strength and light weight of magnesium are desired, but require corrosion resistance and conductivity include portable communications, space exploration and naval applications.
One possible solution is an innovative silane coating described in a copending patent application by the same inventor of the present invention, described in U.S. provisional patent application No. 60/301,147. A solution including a sulfane silane, such as bis-triethoxysilylpropyl tetrasulfane is used to coat an unanodized conductive surface. The silane layer coats the surface, preventing contact with humidity, preventing corrosion. Further, since the silane layer is so thin, the break-through voltage is very low so the workpiece is effectively conductive. Despite the remarkable corrosion resistance of a surface treated using the solution, the corrosion resistance is less than that of some anodized surfaces. In a location where the silane coated surface is repeatedly rubbed or abraded, the silane layer is worn away, exposing untreated surface to the elements, leading to corrosion. Lastly, unlike anodization, the silane layer does not increase the hardness of the surface.
In the art, a number of methods for depositing a conductive layer on magnesium and magnesium alloys are known. Many methods involve the direct application of a nickel layer onto a magnesium surface. Best known is the electroless nickel method where using a multistage electroless process a nickel layer is applied to a copper layer applied to a zinc layer applied to a magnesium workpiece (shorthand: Ni/Cu/Zn/Mg sandwich). Although highly effective in producing a hard, corrosion resistant and conductive workpiece, the method is expensive and is environmentally damaging due to the extensive use of poisonous cyanide compounds.
Ingram & Glass Ltd. (Surrey, United Kingdom) provide an electroless method of applying a Ni/Zn/Mg sandwich. Although conductive and hard, a workpiece so treated corrodes rather easily. Since the nickel and zinc layers are porous, humidity penetrates to the magnesium surface and leads to galvanic corrosion.
ATOTECH (Rock Hill, S.C., USA) and Enthone-OMI (Foxborough, Mass., USA) provide the intensive etching of a magnesium surface with fluorides solutions followed by the electroless application of a conductive nickel layer on the resulting magnesium fluoride (MgF) layer. Although conductive, corrosion resistance is poor. Further, the etching steps damage the surface, especially of die-cast parts, and are thus unsuitable for high-precision workpieces. The ATOTECH method further uses highly toxic and environmentally dangerous chromates.
In addition to the above-discussed disadvantages, all the methods are suitable for application only to an entire workpiece. It is difficult, using the teachings known in the art to fashion a magnesium or magnesium alloy workpiece having a surface where selected areas are conductive whereas the other areas are not conductive.
It would be highly advantageous to have a method for treating magnesium or magnesium alloy surfaces so as to have high corrosion resistance and hard yet conductive surface. Further, it is preferable that such a treatment be selective, that is that after treatment only selected areas of a surface are conductive.